Luminaires offering thin form factor and wide area output are highly useful and widely implemented across a range of different lighting applications. One common application is their use for ceiling lighting, for example in offices and other commercial or public spaces. Here, important design considerations include both the need to generate an output offering low glare, and also the need to provide a luminaire achieving uniform illuminance of visible output surfaces (for aesthetic as well as practical reasons).
Currently, thin form factor and low-glare output can be achieved in state of the art devices, but at the cost of a luminous output which does not cover the entirety of visible output surfaces. This is demonstrated in FIGS. 1 and 2 which illustrate cross-sectional and ‘underside’ views respectively of a state of the art luminaire 12, achieving thin architecture and low-glare.
As shown in FIG. 1, in order to achieve low glare, the luminaire 12 comprises a central reflective element 18 which specularly reflects incident light emitted from the light sources 14 onto the reflective inner surfaces of a housing 20. The central reflective element 18 provides a light mixing function within the interior of the housing and limits the range of output angles at which light may be emitted from the device. However, as shown in FIG. 2, the presence of the central reflective element 18 means that light is output from the device only through outer annular output window 16, leaving a dark circular shadow at the centre of the visible output surface.
A central dark region such as this is avoided in alternative state of the art solutions, whilst still maintaining low-glare. However, this comes at the cost of thicker form factor. One example of such a solution is illustrated in FIG. 3. In order to achieve low-glare, the provided luminaire 22 comprises a parabolic louvre 23 which limits the range of ray output angles so as not to exceed a particular shielding angle. When the louvre is viewed at angles beyond the shielding angle, the visible luminous intensity is greatly reduced, and thus any potential glare diminished or avoided.
However, such a parabolic reflector increases the depth of the provided luminaire, and hence does not provide the ideal solution for applications where thin form factor is an important concern.
Thin form factor and uniform illuminance of visible output surfaces is achievable in many further examples of state of the art devices, but typically at the cost of increased glare. Solutions may include for example the provision of a thin-panel housing comprising a set of light sources arranged directly opposite a diffusive light output window. While a diffuser will limit the worst of any glare, the direct angle at which the light sources face the transmissive output surface means that glare is still increased compared to other solutions which provide light mixing or otherwise limit angular output range.
A final possible known solution is to augment the above-mentioned arrangement with a further optical plate designed to shape the output profile of the emitted light. However, such a system which includes multiple optical elements (diffusive output window and light-output shaping element) is more complex to produce and incurs greater costs.
There is a need therefore for a luminaire capable of achieving thin form factor and low-glare, whilst also providing uniform spread of illuminance across the totality of visible light output surface(s), which may be manufactured with fewer components and at reduced cost.